Pointing devices, such as mice and trackballs, are well known peripherals for personal computers and workstations. Such pointing devices allow rapid relocation of the cursor on a display screen, and are useful in many text, database and graphical programs. Perhaps the most common form of pointing device is the electronic mouse; the second most common may well be the trackball.
With a mouse, the user controls the cursor by moving the mouse over a reference surface; the cursor moves a direction and distance proportional to the movement of the mouse. Although some electronic mice use reflectance of light over a reference pad, and others use a mechanical approach, most prior art mice use a ball which is on the underside of the mouse and rolls over the reference surface (such as a desktop) when the mouse is moved. In such a device, the ball contacts a pair of shaft encoders and the rotation of the ball rotates the shaft encoders, which historically includes an encoding wheel having a plurality of slits therein. A light source, often an LED, is positioned on one side of the encoding wheel, while a photosensor, such as a phototransistor, is positioned substantially opposite the light source. Rotation of the encoding wheel therebetween causes a series of light pulses to be received by the photosensor, by which the rotational movement of the ball can be converted to a digital representation useable to move the cursor.
Although such an approach has worked well for some time, with high quality mice and trackballs providing years of trouble-free use, the mechanical elements of such pointing devices necessarily limit the useful life of the device.
Conventional optical mice which illuminate a reference pad, while having few or no mechanical parts, have historically been limited due to the need for the reference pad to have a regular pattern, as well as many other limitations.
While conventional optical mice have typically required a reference pad, two methods are known in the general optical art for detecting movement of a scattering surface illuminated by coherent illumination. The first such approach employs illumination of the surface with two light sources and using a single detector; the second includes illumination with only a single beam but using a grating filter in front of a single detector. In both these cases, forward and backward movement cannot be distinguished, in what is referred to as sign ambiguity. Likewise, in both cases the detection is sensitive to one direction of movement in the plane. Further, in the case of the first approach, the two illuminating beams have to be rotated to be sensitive to another direction of movement; that is, for each direction of movement an independent detection system of illuminating beams and detector has to be used. In the case of the second approach, the grating filter in front of the detector has to be rotated to be sensitive to another direction of movement.
The subject matter of co-pending U.S. patent application Ser. No. 09/039,164, filed Mar. 13, 1998, entitled "Method For Generating Quasi-sinusoidal Signals," overcomes substantially all of the foregoing limitations identified above, and provides an improved system and method for optical detection of motion of a detector relative to an irregularly speckled or patterned surface. The optical detection is accomplished by detecting zero voltage crossings of quasi-sinusoidal signals generated by detectors in response to, for example, an optical mouse being moved over a reflective surface.
Under certain conditions, the quasi-sinusoidal signals can be corrupted by noise from, for example, non-uniform beam illumination or non-uniform surface reflectance. Non-uniform illumination occurs if the reflective surface is illuminated by a light source having a non-uniform distribution, i.e., the light intensity measured across the illumination spot is not constant. Non-uniform surface reflectance occurs if a detector is moved along a non-uniform surface, such as a printed pattern or textured desk surface. For example, noise can be generated by moving the detector over a surface having black letters on a white surface, such as found on the page of a book. The noise manifests itself as a time varying low frequency offset to the quasi-sinusoidal signals, which prevents the accurate detection and counting of zero crossings. The use of zero crossings to determine cursor displacements is described in detail in U.S. Pat. No. 5,729,009, which is incorporated by reference herein in its entirety.
Although using a reference pad with a uniform pattern may reduce noise from non-uniform surface reflection, there are many applications where the reference pad may be too burdensome or inconvenient to use. For example, computer users often work in environments where there are no flat surfaces to place the reference pad on, such as airplanes or cars. Moreover, many computer retailers prefer to place their logos or brand names on reference pads for promotional purposes. These logos or brand names, in essence, generate non-uniform surface reflectance.
Accordingly, there is a need for a system and method for generating band-limited quasi-sinusoidal signals for use in, for example, an optical mouse, to enable the user to move the mouse over almost any surface, including those surfaces having printed material or texture. Such a system and method should not add significant hardware or manufacturing costs to existing optical mouse designs. Nor should such a system and method significantly impact the size or weight of existing optical mouse designs.